FIG. 5 shows a motor structure of prior art 1, and this motor is formed of stator 40 and rotor 50. Stator 40 comprises the following elements: stator core 41, winding 43 wound via insulator 42 on respective teeth of stator core 41 in a concentrated manner, terminal 44, circuit board 45, resin mold 46 for sealing all or parts of the elements discussed above, and bracket 47. Three-phase windings are regularly used in this motor. Circuit board 45 includes various components necessary for driving the motor.
Rotor 50 is placed inside of stator 40, and has shaft 53 at the center of rotor core 52. Shaft 53 is rotatably supported by two bearings 54. Rotor 50 forms a surface magnet rotor, i.e. rotor magnet 51 is mounted on an outer wall of rotor core 52. Rotor 50 rotates on shaft 53 due to the interaction between the magnetic field formed by an electric current running through winding 43 wound on stator 40 and the magnetic poles of rotor magnet 51.
Driving of the motor needs to regulate the electric current running through winding 43 in response to a rotational position of rotor 50, so that some means is needed for accurately sensing the rotational position of rotor 50.
In a conventional manner, position sensor 61, such as a Hall element or a Hall IC, mounted to circuit board 45 is used in many cases as a means for sensing the rotational position for the motor to sense the rotational position of rotor 50.
At this time, extra-close placement of rotor magnet 51 mounted on rotor 50 to position sensor 61 allows sensing a major magnetic flux, which rotates rotor 50, generated from rotor magnet 51. However, in actual, presence of a coil end, namely, a protruding part of winding 43 from stator core 41, sets a limit to the closer placement of rotor magnet 51 to position sensor 61. The extra-close placement of rotor magnet 51 to position sensor 61 also refers to the close placement of position sensor 61 to stator core 41 or winding 43. As a result, position sensor 61 is affected by the magnetic flux, which is generated by the electric current running through winding 43 and issued from stator 40, so that the accuracy of sensing the rotor position is lowered, for position sensor 61 originally aims to sense the rotor position by sensing the magnetic flux issued from rotor magnet 51.
FIG. 6 shows a motor structure of prior art 2. Elements similar to those of prior art 1 shown in FIG. 5 have the same reference marks, and the descriptions thereof are omitted here. The motor of prior art 2 differs from the motor of prior art 1 in the following point: As shown in FIG. 6, position sensing magnet 62 is mounted to rotor 50 besides rotor magnet 51. Position sensing magnet 62 is axially magnetized so that the magnetic flux generated from magnet 62 can positively interlink with position sensor 61, which thus accurately senses a rotational position of rotor 50. This structure is disclosed in, e.g. Unexamined Japanese Patent Publication No. H11-299207.
The motor of prior art 2, however, has discrete components of rotor magnet 51 and position sensing magnet 62, so that variations are obliged to happen in the positional relation between magnet 51 and magnet 62 both mounted to rotor core 52. The variations sometimes lower the accuracy of sensing the position, and the structure discussed above increases the number of steps of assembling rotor 50.